Sequential activation of electrical apparatus

ABSTRACT

A time delay device consists of a pair of spaced conductors a conductive link made from a low melting material located between the two conductors and in contact with one of the conductors, and a heating resistor to cause the link after a predetermined time to melt and bridge across the two conductors.

United States Patent [191 1 11 3,836,884 Oliver 1 1 Sept. 17, 1974 [54] SEQUENTIAL ACTIVATION 0F 2,085,379 6/1937 Metzgar 337/182 ELECTRICAL APPARATUS 2,501,457 3/1950 Thelin 337/415 2,779,014 1/1957 Mounce 337/145 X [75] Inventor: George Albert Oliver, 2,832,265 4/1958 Reid et a1. 337 145 Johannesburg, South Africa 3,386,063 5/1968 Mansfield 317/15 [73] Assignees: Exec (Proprietary) Limited; AE &

CI Limited, Johannesburg, South Prlmary Examiner-l Miller Africa; interest to each Assistant Examiner-Fred E. An ,A t, F C h 22 Filed: Jan. 18, 1973 3133 gen or man & [21] Appl. No.: 324,569

[30] Foreign Application Priority Data [5 7] ABSTRACT Jan. 19, 1972 South Africa 72/0375 A time delay device consists of a pair of spaced conductors a conductive link made from a low melting [52] US. Cl 337/163, 337/182, 337/184 material located between the two conductors and in Int. CL ontact one of the onductors and a heating re- Field Of Search 337/163, 182, 183, 145, sistor to cause the link after a predetermined time to 337/141 melt and bridge across the two conductors.

[56] References Cited 16 Claims, 3 Drawing Figures UNITED STATES PATENTS 1,829,149 10/1931 I Mason et a1. 337/415 SEQUENTIAL ACTIVATION OF ELECTRICAL APPARATUS The present invention relates to the sequentialactivation of electrical apparatus and more particularly to the sequential ignition of fused detonators usedin blasting operations.

' lays and affects production. Electrical systems presently in use feature complicated switching mechanisms and have some form of external control box which controls the timing operation.

It is an object of the present invention to provide a novel time delay device which is suited for use in the sequential activation of electrical apparatus and more particularly the sequential ignition of fused detonators used in blasting.

' A time delay device according to the invention includes a pair of spaced conductors, a conductive link between the spaced conductors and in contact with one of the conductors, and means for heating the link to cause it, after a predetermined time, to melt and bridge across the two conductors.

The conductive link may be constituted by a portion of one of the conductors or it may be a discrete element formed separately or integrally with one of the conductors.

Further according to the invention, theconductors are suitably formed by concentric cylinders and the link comprises an'annulus between the two cylinders. A body of flux is preferably located on the free surface of the annulus to improve its flow properties when molten. By free surface" is meant that surface which is out of contact with the conductors.

Still further according to the invention the heat may be applied to the link through'one of the conductors. The link may be made of any suitable material but is preferably a low melting material and particularly a low melting metal or metal alloy.

Conveniently the conductors may be of any suitable metal for example brass, aluminium and copper.

In the preferred form of the invention however one I conductor is formed by a sleeve and the other by a central core contained at least partially within the sleeve. The conductive link is in the form of an annulus which surrounds the core in spaced relation and which is in electrical contact with the sleeve. The core is preferably formed by one of the leads of a heating resistor housed within the sleeve.

A novel system for the sequential activation of a series of electrical devices incorporating the time delay devices is also provided.

The system for the sequential activation of a series of electrical devices such as detonator fuse heads, includes a series of electrical devices each of which is connected into a high resistance circuit incorporating a time delay device of the invention, the time delay devices being adapted to be triggered sequentially to allow an increased current to flow sequentially through the series of electrical devices to activate them, the activation of each electrical device in the series being adapted to apply current to the high resistance circuit of the next electrical device in the series.

In the accompanying drawings which illustrate the invention by way of example.

FIG. 1 is a section through a time delay device accoring to the invention,

FIG. 2 is a similar view through another embodiment of the invention, and

FIG. 3 is a circuit diagram'of a system for sequentially igniting blasting fuses which utilises the time delay device of FIG. 1 or FIG. 2.

The time delay device shown consists of a heating element in the form of a resistor 10 which is encapsulated in an insulating compound 12 for example an epoxy resin, contained within an inner cylinder 14 made of a conducting material, for instance brass.

One external connection 16 is connected to one side of the resistor 10 and to the cylinder 14 through its closed end 18. The other external connection 20 is connected to the other side of the resistor 10.

Two insulators 22 are located at each end of the cylinder l4 and an annulus 24 composed of a low melting alloy, such as Woods Alloy, is located between the insula'tors 22 about the cylinder 14. An outer cylinder 26, also composed of a conductive material e.g. brass, forms an enclosure for the assembly. On the free surface of the annulus 24 there is a body of flux 27. A further ex'ternall connection 28 is connected to the outer cylinder 26. v

The external diameter of the annulus 24 is smaller than the external diameter of the insulators 22 and thus it will be noted that the annulus 24 is spaced from the outer cylinder 26 '(i.e. is out of contact with it). The cylinder 26 is encapsulated in a body of thermal and electrical insulating material.

When a voltage is applied across the external connections 16 and 20, the heat which is developed by the resistance 10 causes the annulus 24 after a predetermined time interval, to melt and come into physical contact with the outer cylinder 26, thus completing an electrical circuit between the inner cylinder 14 and the outer cylinder 26. The device thus functions as a time delay.

In the embodiment shown in FIG. 2 the device consists-of a brass sleeve 30 into which has been inserted a resistor 31. The resistor 31 is suitably a tight fit within the sleeve 30 and its two external connections or wires 32 and 33 project as shown from the ends of the sleeve 30.

Located within the sleeve, preferably as a snug fit, is an annulus 34 of Woods Alloy. Between the annulus 34 and the end of the resistor body 38 is a fibre washer 40 which serves to insulate the annulus 34 from the resistor body 38 and also effectively to prevent the wire 33 from being moved into contact with the annulus 34. A further external connection 42 is connected as shown to the sleeve 30. A layer of flux 43 is provided on the wire 33.

The sleeve 30- is encapsulated in a covering of a suitable electrical and thermal insulating material.

After a predetermined time, a voltage applied across the resistor 31 will cause the annulus 34 to melt thereby bridging across the sleeve 30 and the external connection-33.

Referring to FIG. 3 there is shown a circuit for a system which can be used to sequentially ignite a series of fused detonators. In the circuit diagram the fuse heads of the detonators are indicated at 50, 52 and 54 and the time delay devices at 56, 58 and 60. Each time delay device consists of one of the devices shown in either FIG. 1 or FIG. 2.

When voltage is applied across the terminals 44 and 45 current passes through the resistor or 31 in the first time delay device 56 and through the first fusehead 50. Since the resistance of the fusehead 50 is low compared to that of the resistor 10 or 31, it acts as an effective shorting link across the second time delay device 58. The current passing through the fuse head 50 is not sufficient to ignite the fusehead 50.

However, after a predetermined delay, which is proportional to the applied voltage, the heat generated by resistor 10 or 32 melts the low melting alloy annulus 24 or 34 to complete an electrical circuit between the inner cylinder 14 and the outer cylinder 26 or in the FIG. 2 embodiment between the wire 33 and the sleeve 30. As a result the resistor 10 or 32 is effectively shorted out putting the full voltage across the fuse head 50.

This increased voltage is sufficient to initiate the fuse-head 50 which ignites the fuse of the blasting detonator. The fuse-head 50 simultaneously becomes open circuit.

The subsequent time delay device 58 is not energised by the applied voltage and the cycle of operation repeated to ignite the following fuse.

I claim:

1. A time delay device including first and second spaced conductors, a fusible element in contact with at most the first conductor, electrical heating means in series with the first conductor for heating the fusible element, on application of current to the first conductor, to cause the fusible element after a preselected time too melt and short circuit the conductors thereby establishing a current path through the second conductor.

2. A time delay device as claimed in claim 1 in which the fusible element is a discrete element arranged between the two conductors.

3. A time delay device as claimed in claim 1 in which the electrical heating means includes a resistor.

4. A time delay device as claimed in claim 3 in which the resistor is encapsulated in an insulating material.

5. A time delay device as claimed in claim 1 in which the conductors include a pair of concentric cylinders and the fusible element comprises an annulus between the two cylinders.

6. A time delay device as claimed in claim 5 including flux on the free surface of the annulus to improve its flow properties when molten.

7. A time delay device as claimed in claim 5 in which the annulus embraces the inner cylinder of the pair of cylinders.

8. A time delay device as claimed in claim 7 further including two insulators between the ends of the cylinders for spacing the cylinders from each other.

9. A time delay device as claimed in claim 1 in which the first conductor includes a sleeve and the second conductor a central core within the sleeve and the fusible element is an annulus surrounding the core in spaced relation and in electrical contact with the sleeve.

10. A time delay device as claimed in claim 9 in which the electrical heating means includes a resistor which is contained within the sleeve and the core is one of the external electrical connections of the resistor.

11. A time delay device as claimed in claim 10 in which the annulus is a fit within the sleeve.

12. A time delay device as claimed in claim 1 in which the conductors are of metal.

13. A time delay device as claimed in claim 1 in which the conductive link is composed ofa low melting metal or metal alloy.

14. A time delay device as claimed in claim 13 in which the low melting alloy is Wood's Alloy.

15. A system for the sequential activation of a series of electrical devices including a series of electrical devices each of which is connected into a high resistance circuit incorporating a time delay device according to claim 1, the time delay devices being adapted to be triggered sequentially to allow an increased current to flow sequentially through the series of electrical devices to activate them, the activation of each electrical device in the series being adapted to apply current to the high resistance circiut of the next electrical device in the series.

16. A time delay device as claimed in claim 5 in which the inner cylinder is part of the first conductor. l k 

1. A time delay device including first and second spaced conductors, a fusible element in contact with at most the first conductor, electrical heating means in series with the first conductor for heating the fusible element, on application of current to the first conductor, to cause the fusible element after a preselected time too melt and short circuit the conductors thereby establishing a current path through the second conductor.
 2. A time delay device as claimed in claim 1 in which the fusible element is a discrete element arranged between the two conductors.
 3. A time delay device as claimed in claim 1 in which the electrical heating means includes a resistor.
 4. A time delay device as claimed in claim 3 in which the resistor is encapsulated in an insulating material.
 5. A time delay device as claimed in claim 1 in which the conductors include a pair of concentric cylinders and the fusible element comprises an annulus between the two cylinders.
 6. A time delay device as claimed in claim 5 including flux on the free surface of the annulus to improve its flow properties when molten.
 7. A time delay device as claimed in claim 5 in which the annulus embraces the inner cylinder of the pair of cylinders.
 8. A time delay device as claimed in claim 7 further including two insulators between the ends of the cylinders for spacing the cylinders from each other.
 9. A time delay device as claimed in claim 1 in which the first conductor includes a sleeve and the second conductor a central core within the sleeve and the fusible element is an annulus surrounding the core in spaced relation and in electrical contact with the sleeve.
 10. A time delay device as claimed in claim 9 in which the electrical heating means includes a resistor which is contained within the sleeve and the core is one of the external electrical connections of the resistor.
 11. A time delay device as claimed in claim 10 in which the annulus is a fit within the sleeve.
 12. A time delay device as claimed in claim 1 in which the conductors are of metal.
 13. A time delay device as claimed in claim 1 in which the conductive link is composed of a low melting metal or metal alloy.
 14. A time delay device as claimed in claim 13 in which the low melting alloy is Wood''s Alloy.
 15. A system for the sequential activation of a series of electrical devices including a series of electrical devices each of which is connected into a high resistance circuit incorporating a time delay device according to claim 1, the time delay devices being adapted to be triggered sequentially to allow an increased current to flow sequentially through the series of electrical devices to activate them, the activation of each electrical device in the series being adapted to apply current to the high resistance circiut of the next electrical device in the series.
 16. A time delay device as claimed in claim 5 in which the inner cylinder is part of the first conductor. 